Leaders of the six Gulf Arab countries forming the Gulf Co-operation Council (GCC) are due to meet in Riyadh to discuss progress in their overall co-operation since their summit in December, when the Saudi monarch urged for a "transition from the stage of co-operation to the Union."

The summit on Monday is expected to discuss the Saudi proposal to develop their six-nation council into a union, possibly starting with a merger between Saudi Arabia and Bahrain.

The exact nature of this union remains unclear but Bahrain's state minister for information, Samira Rajab, said it could follow the "European Union model".

"The summit will discuss all the points, including the points of union," said Bahraini Foreign Minister Khalid bin Ahmed Al Khalifa after preparatory talks in Riyadh on Sunday.

A committee made up of three representatives from each of the six member states - the United Arab Emirates, Saudi Arabia, Bahrain, Kuwait, Qatar and Oman - will submit its findings on the proposed union to Monday's summit.

The discussions will be closely watched on the streets of Bahrain where the main opposition has denounced the idea of greater union.

Sheikh Ali Salman, the leader of Bahrain's main opposition formation, Al-Wefaq, has criticised the project which he said must first be subjected to a referendum that should take place in all GCC states.

"Bahrain gained its independence [in 1971] following a referendum" overseen by the United Nations, said Salman in a speech on Sunday.

"The people of Bahrain alone have the right to" decide, he said, adding the kingdom's ruling "Al-Khalifa (dynasty) has no right to decide a union or confederation with any country".

Bahrain has been wracked by unrest for more than 15 months that activists say has killed 81 people. Bahrain's government says demonstrators were responsible for many of the killings.